1. Field of the Invention
The present invention relates to an image display device and a control method thereof.
2. Description of the Related Art
Industrial displays such as color management displays include models equipped with a color adjusting function that is referred to as color correction (calibration) for correcting color shift due to individual variability or deterioration of a display and displaying color in an accurate manner. Soft proofing is a process which uses such color management displays that accurately display color to perform color correction and verification of a printing result on the display. Recent improvements in color reproducibility, color accuracy, and definition of displays have made soft proofing which simulates a final printing finish on a display more practical.
When a person looks at printed matter or a display and senses color, how color is sensed is known to differ depending on a size (visual angle) of an observed object projected onto the eyes. For example, when an observer views two color chips of a same color but with different sizes, the colors of the two color chips are sensed as different colors. This phenomenon is referred to as an “area effect of color”. The “area effect of color” is conceivably caused by a change in visual cell sensitivity or, in other words, a spectral sensitivity curve (also referred to as a color matching function) of the eyes in accordance with visual angles. A color (stimulus value) as sensed by a person is determined by the multiplication of a spectral spectrum of an object and a color matching function. Since color matching functions change when viewing color chips with different sizes, the colors sensed by an observer differ even when the color chips share the same color. The International Commission on Illumination (abbreviated as CIE) defines two color matching functions: a color matching function for a 2-degree visual angle (CIE 1937 standard colorimetric observer) and a color matching function for a 10-degree visual angle (CIE 1964 supplementary standard colorimetric observer).
A mechanism by which a person senses color that is displayed on a display and problems arising therefrom will now be described. Visual cells in a human eye include cone cells which sense colors of red, green and blue and which differ from one another in spectral sensitivity. Accordingly, a person perceives color by adding up magnitudes of sensations of red, green and blue in the brain. A curve that is traced by respective sensitivities (spectrum stimulus values) of red, green and blue of the eye with respect to an equal energy spectrum is referred to as a color matching function. The sensitivity of red is denoted by x(λ), the sensitivity of green by y(λ), and the sensitivity of blue by z(λ). FIG. 3 shows color matching functions of a 2-degree visual field and a 10-degree visual field. The color matching function of the 2-degree visual field is expressed as x2(λ), y2(λ), z2(λ), and the color matching function of the 10-degree visual field is expressed as x10 (λ), y10(λ), z10(λ). Ultimately, color as sensed by a person is expressed by tristimulus values of a CIE XYZ color system calculated from the color matching function x(λ), y(λ), z(λ). Among the XYZ stimulus values, X denotes a stimulus with respect to red, Y denotes a stimulus with respect to green, and Z denotes a stimulus with respect to blue. FIG. 4 is a diagram illustrating a mechanism by which a person senses color. A top left diagram shows a spectral spectrum s(λ) when a white color chip is displayed on a display having a backlight constituted by RGB LEDs. A bottom left diagram shows the color matching function x(λ), y(λ), z(λ). The three diagrams on the right show stimuli of red, green and blue as sensed by a person. Each stimulus is calculated by integrating a multiplication of a spectral spectrum and the color matching function. In this case, a sum of the stimuli is perceived as white.
Expression 1 represents an XYZ calculation formula for a 2-degree visual field and Expression 2 represents an XYZ calculation formula for a 10-degree visual field, where k denotes a coefficient.
                              X          =                      k            ⁢                                          ∑                λ                                                                              ⁢                              (                                                                            x                                              2                        ⁢                        °                                                              ⁡                                          (                      λ                      )                                                        ×                                      s                    ⁡                                          (                      λ                      )                                                                      )                                                    ⁢                                  ⁢                  Y          =                      k            ⁢                                          ∑                λ                                                                              ⁢                              (                                                                            y                                              2                        ⁢                        °                                                              ⁡                                          (                      λ                      )                                                        ×                                      s                    ⁡                                          (                      λ                      )                                                                      )                                                    ⁢                                  ⁢                  Z          =                      k            ⁢                                          ∑                λ                                                                              ⁢                              (                                                                            z                                              2                        ⁢                        °                                                              ⁡                                          (                      λ                      )                                                        ×                                      s                    ⁡                                          (                      λ                      )                                                                      )                                                                        [                  Expression          ⁢                                          ⁢          1                ]                                                      X            ′                    =                      k            ⁢                                          ∑                λ                                                                              ⁢                              (                                                                            x                                              10                        ⁢                        °                                                              ⁡                                          (                      λ                      )                                                        ×                                      s                    ⁡                                          (                      λ                      )                                                                      )                                                    ⁢                                  ⁢                              Y            ′                    =                      k            ⁢                                          ∑                λ                                                                              ⁢                              (                                                                            y                                              10                        ⁢                        °                                                              ⁡                                          (                      λ                      )                                                        ×                                      s                    ⁡                                          (                      λ                      )                                                                      )                                                    ⁢                                  ⁢                              Z            ′                    =                      k            ⁢                                          ∑                λ                                                                              ⁢                              (                                                                            z                                              10                        ⁢                        °                                                              ⁡                                          (                      λ                      )                                                        ×                                      s                    ⁡                                          (                      λ                      )                                                                      )                                                                        [                  Expression          ⁢                                          ⁢          2                ]            
A significant difference between the values of XYZ and X′Y′Z′ means that, when viewing a color chip displayed on a display, the color chip is recognized as colors that differ between the 2-degree visual field and the 10-degree visual field. How different the colors appear to a human eye is expressed by a color difference ΔE. ΔE denotes a Euclidean distance calculated by converting CIE XYZ into a CIE Lab color space (refer to Expression 3). A ΔE value of around 1.2 represents a range in which color chips placed side by side can be identified as a same color.[Expression 3]ΔEab=√{square root over ((ΔL)2+(Δa)2+(Δb)2)}{square root over ((ΔL)2+(Δa)2+(Δb)2)}{square root over ((ΔL)2+(Δa)2+(Δb)2)}   (Expression 3)
With soft proofing in which a final printing finish is verified on a display, stability and accuracy of color is important. Desirably, a color does not change with a magnitude (visual angle) of appearance of the color. On the other hand, there is also a need to expand a color gamut to be reproduced of the display for viewing photographs or the like rather than color stability. A color gamut to be reproduced of a liquid crystal display is determined by three factors including a spectral spectrum of a backlight, spectral transmittance characteristics of a color filter, and a spectral spectrum of a liquid crystal panel. Expanding a color gamut to be reproduced of a liquid crystal display requires preventing the occurrence of color mixing between dominant wavelengths of the respective primary colors of red, green and blue and other sub-peaks obtained by integrating a spectral spectrum of a color matching function and a spectral spectrum of the display. In other words, a color gamut to be reproduced is expanded by increasing color purity of the respective primary colors of red, green and blue. However, as shown in FIG. 14, a spectral spectrum of a cold cathode fluorescent lamp (CCFL) that is widely used as a light source of a conventional liquid crystal panel includes many sub-peaks in addition to dominant wavelengths. Consequently, color mixing occurs and results in a narrow color gamut to be reproduced. Similarly, with a spectral spectrum of a white light-emitting diode shown in FIG. 15, while the spectral spectrum does not include sub-peaks as in the case of a CCFL, the spectral spectrum spreads over a wide wavelength region. Consequently, color mixing occurs and results in a narrow color gamut to be reproduced.
In consideration thereof, in recent years, RGB LEDs are sometimes used as a backlight of a liquid crystal display. Using RGB LEDs realizes a wider color gamut as compared to using a cold cathode fluorescent lamp (CCFL). Japanese Patent Application Laid-open No. 2007-264659 proposes a technique for switching a light source of a backlight between light-emitting diodes of red, green, and blue and a white light-emitting diode depending on a display image quality mode of a display device. Japanese Patent Application Laid-open No. 2007-264659 describes that a wider color gamut to be reproduced is produced by using red, green, and blue light-emitting diodes as compared to using a white light-emitting diode as a light source of a backlight.
In addition, generally, there are methods of reducing a difference in color appearance by calibrating color reproduction of a display for viewing in a 2-degree visual field and viewing in a 10-degree visual field. Japanese Patent Application Laid-open No. 2006-253502 discloses reducing a change in color by preparing a plurality of LEDs with different peak wavelengths for each of RGB and adopting a composite spectrum as a spectrum of each color. By determining a composite spectrum of LEDs according to color matching functions of a 2-degree visual field and a 10-degree visual field, a difference in appearance due to an area effect of color can be reduced.